1. Field of the Invention
The field of the invention is magnetic resonance imaging (MRI) and in particular local coils for use in receiving MRI signals.
2. Background Art
In MRI, a uniform magnetic field B.sub.0 is applied to an imaged object along the z axis of a Cartesian coordinate system, the origin of which is approximately centered within the imaged object. The effect of the magnetic field B.sub.0 is to align the object's nuclear spins along the z axis.
In response to a radio frequency (RF) excitation signal of the proper frequency, oriented within the x-y plane, the nuclei precess about the z-axis at their Larmor frequencies according to the following equation: EQU .omega.=.gamma.B.sub.0
where .omega. is the Larmor frequency, and .gamma. is the gyromagnetic ratio which is constant and a property of the particular nuclei.
Water, because of its relative abundance in biological tissue and the properties of its nuclei, is of principle concern in such imaging. The value of the gyromagnetic ratio .gamma. for water is 4.26 kHz/gauss and therefore in a 1.5 Tesla polarizing magnetic field B.sub.0 the resonant or Larmor frequency of water is approximately 63.9 MHz.
In a typical imaging sequence, the RF excitation signal is centered at the Larmor frequency .omega. and applied to the imaged object at the same time as a magnetic field gradient G.sub.z is applied. The gradient field G.sub.z causes only the nuclei in a slice through the object along a x-y plane to have the resonant frequency .omega. and to be excited into resonance.
After the excitation of the nuclei in this slice, magnetic field gradients are applied along the x and y axes. The gradient along the x axis, G.sub.x, causes the nuclei to precess at different frequencies depending on their position along the x axis, that is, G.sub.x spatially encodes the precessing nuclei by frequency The y axis gradient, G.sub.y, is incremented through a series of values and encodes y position into the rate of change of phase of the precessing nuclei as a function of gradient amplitude, a process typically referred to as phase encoding.
A weak nuclear magnetic resonance generated by the precessing nuclei may be sensed by the RF coil and recorded as an NMR signal. From this NMR signal, a slice image may be derived according to well known reconstruction techniques. An overview NMR image reconstruction is contained in the book "Magnetic Resonance Imaging, Principles and Applications" by D. N. Kean and M. A. Smith.
The quality of the image produced by MRI techniques is dependent, in part, on the strength of the NMR signal received from the precessing nuclei. For this reason, it is known to use an independent RF receiving coil placed in close proximity to the region of interest of the image object to improve the strength of this received signal. Such coils are termed "local coils" or "surface coils". The smaller area of the local coil permits it to accurately focus on NMR signal from the region of interest. Further, such local coils may be of higher quality factor or "Q" than the RF transmitting coil increasing the selectivity of the local coil and the relative strength of the acquired NMR signal.
The smaller size of the local coil makes it important that the local coil be accurately positioned near the region of interest. For "whole volume" coils, employing two antenna loops to receive the NMR signal from a volume defined between the loops, accurate positioning of the coils is particularly important. For a whole volume shoulder coil, for example, the two antenna loops must be placed on opposite sides of the shoulder but generally opposed along a single axis. Further, the relative positions of the coils must be adjustable to accommodate a range of shoulder dimensions. Adjustment of the coil is preferably simple and rapid so as not to prolong unduly the time required to perform the MRI scan.
A major technical problem in NMR systems is "decoupling" the local coil from the RF excitation signal during the application of the RF excitation signal. Such decoupling reduces the distortion of the excitation field by the local coil and prevents potential damage to the sensitive circuits connected to the local coil from large induced voltages.
Inductive coupling between the excitation field and the local coil may focus the deposition of the RF energy on a reduced volume the imaged object. In the case of the medical imaging of a patient, such focused energy may cause burns. Energy coupled to the local coil itself may cause heating of that coil, producing indirect burns to the patient and damage to the local coil and its circuitry. The problem of distortion and inductive coupling is compounded by the typically high Q of the local coils.
One method of decoupling the local coil from the RF excitation field is through the use of one or more solid state switches positioned along the local coil which may be activated either by an external electrical signal or by the RF excitation field itself. These switches disable or detune the local coil. One such approach which shows generally the use of back-to-back diodes for decoupling a local coil is described in U.S. Pat. No. 4,725,779, issued Feb. 16, 1988 to Hyde et al., entitled: "NMR Local Coil with Improved Decoupling" and hereby incorporated by reference.
The semiconductor elements used for decoupling are subject to failure from mechanical damage, thermal stress and aging. Generally it cannot be predicted with any certainty whether a semiconductor will fail as a short circuit or as an open circuit. Depending on the design of the coil, one such failure mode will leave the local coil coupled to the RF excitation field.
The local coil is typically connected to the MRI system by a long cable permitting the coil to be placed first on the patient and the patient then to be positioned within the bore of the polarizing magnet used to generate field B.sub.0. Once in place, the local coil may be tuned remotely by means of varactor diodes or similar devices adjusted by a remotely located DC biasing voltage carried on separate conductors in the attaching cable. The ability to tune a local coil may improve the image quality, however the additional conductors needed for the biasing voltage render the connecting cable less flexible and more difficult to position.